1. Field of the Invention
The present invention relates to a mask and a projection exposure method utilizing said mask, and more particularly to a photomask for semiconductor device manufacture.
2. Related Background Art
With the recent size reduction in the geometry of patterns of the semiconductor integrated circuits, there has been encountered a drawback, in the projection exposure apparatus, of thermal deformation of the exposure mask (hereinafter simply called "mask") resulting from the absorption of the exposure light and giving rise to a variation in the imaging characteristics.
Particularly in the conventional mask, in which the patterns to be transferred are formed with chromium (chromium oxide film) on the glass substrate, for example heat absorption is higher in the chromium portion constituting the patterns than in the glass portion having higher transmittance to the light. Such heat absorption in the chromium portion has tended to increase in recent years, because a technology to reduce the reflectance of the chromium on the mask has been adopted for avoiding flare in the optical system.
Also, the chromium patterns on the mask are not necessarily distributed uniformly on the mask, but may be localized in the distribution. Such localized distribution may result in localized temperature rise on the mask, eventually leading to anisotropic thermal deformation (non-point symmetrical thermal deformation). Such anisotropic thermal deformation may also result in case only a part of the mask is exposed, as, for example, with a light shielding member (variable field diaphragm). Such anisotropic thermal deformation of the mask may cause an anisotropic distortion in the projected image. For compensating the variation in the imaging characteristics in such case, the correction only for the variation in image magnification is insufficient, and there will also be required the correction for example for the variation in distortion, for example.
As explained in the foregoing, the conventional mask, generating the anisotropic distortion in the projected image because of the thermal deformation, requires not only the correction for the variation in image magnification but also the correction for the variation in distortion etc. The variation in image magnification can in general be securely corrected by the displacement of the optical elements in the projection optical system, but the variation in distortion may be not completely correctable, depending on the mode of deformation of the mask, and generally requires a complex method of correction.